As displays for television, personal computer, etc., active-matrix liquid crystal display devices have been used which are capable of high-quality video display. Liquid crystal display devices include pixel formation portions each being provided with a thin film transistor (hereinafter, referred to as a “TFT”) and a pixel electrode. In the case where the TFT is on, when a potential corresponding to video to be displayed is applied to the pixel electrode through a video signal line via the TFT, a voltage (gate-off voltage) for turning off a gate of the TFT is applied to the gate until another potential corresponding to the next video to be displayed is applied. As a result, the TFT is maintained in off state until the next potential is applied, so that the potential corresponding to video to be displayed is held in the pixel formation portion.
However, if the liquid crystal display device, which has a liquid crystal panel provided therein, is subjected to a long period of conduction, TFTs experience a change in off characteristics. As a result, in the case of, for example, a normally black liquid crystal panel (which appears black when no voltage is applied) having N-channel TFTs formed thereon, when a gate voltage is raised from the level of the gate-off voltage, the luminance of video displayed in white is reduced and the video appears as if it is displayed in gray. The gate voltage when the video appears as if it is displayed in gray is called a blurring voltage. The blurring voltage falls as a period of conduction increases, and stops falling when it reaches a predetermined value. In this case, the gate-off voltage needs to be set considering the fall of the blurring voltage, resulting in inconveniences such as the need to enhance a withstanding voltage of a gate driver.
The following are possible reasons why the blurring voltage falls as the period of conduction to the liquid crystal display device increases. Specifically, as the period of conduction increases, a charge is accumulated in the vicinity of a TFT channel region, and an inversion layer is formed in the channel region due to the accumulated charge. As a result, a leakage current flows through the inversion layer formed in the TFT, which is supposed to be in off state. This results in a reduction of the potential corresponding to video to be displayed, which is held in the pixel formation portion, so that the luminance of the video falls. Also, as the period of conduction increases, the amount of accumulated charge increases correspondingly, and therefore the blurring voltage falls, facilitating flow of the leakage current.
Japanese Laid-Open Patent Publication No. 9-152628 describes the fall of the blurring voltage being suppressed by forming a conducting film above the TFT channel region via an interlayer insulating film.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 9-152628